If you're running a very old PC but have managed to persuade Windows 11 to boot, it looks like the rug could soon be pulled from under you, judging by a post claiming that Microsoft's code will now require an instruction not found on old CPUs.

The [1]instruction in question is POPCNT , which first turned up in Intel Core processors toward the end of 2008 as part of the SSE4.2 instruction set extension and AMD's Barcelona architecture in 2007.

POPCNT counts the number of bits in a machine word that have been set (or different from zero.) You might see it in cryptography and it has been lurking in CPU architectures for years, pre-dating Intel and AMD's implementation by decades.

[2]

Its use here means that if your CPU is 15 years old – or older – there is every chance that the next major release of Windows 11, expected to be 24H2, won't boot. However, to put this in context, the instruction arrived in the first generation of Core i5 and i7 processors, which used the Nehalem microarchitecture.

[3]

[4]

The infamous [5]Windows 11 hardware compatibility list excludes pretty much anything earlier than eighth-generation Intel chips, so the discovery is unlikely to have much in the way of mainstream impact. However, customers using workarounds to dodge the Windows 11 hardware check would do well to ensure their hardware is not too old before attempting an install.

Then again, running Microsoft's latest OS on hardware more than a decade old is probably likely being done by enthusiasts than an ordinary users.

[6]

The report of the requirement, discovered by X user @TheBobPony in recent Windows 11 24H2 builds, comes a month after some Windows 10 users found themselves [7]unable to open some Windows apps following an update.

[8]Microsoft's Notepad goes from simple text editor to Copilot conspirator

[9]Closure of Windows 10 upgrade path still catching users by surprise

[10]Microsoft embraces its inner penguin as sudo sneaks into Windows 11

[11]Windows 11 24H2 is coming so we can all shut up about Windows 12 for another year

Some users speculated that a Microsoft engineer had accidentally enabled some newer CPU instructions during compilation, effectively dropping support for several older processors.

The same thing might have happened this time, although it would seem reasonable for Microsoft not to bother with CPUs that are not on the Windows 11 hardware requirements list. Whether that hardware requirements list is reasonable is, however, a whole different discussion.

We contacted the company to ask and will update this article should it respond.

In the meantime, the latest finding is a reminder that even Microsoft – famed for backward compatibility and keeping old kit going – is getting serious regarding hardware requirements. ®

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[1] https://twitter.com/TheBobPony/status/1756771051634135277

[2] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/oses&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2ZcugOKkj@KBlRikOhxKijwAAAQA&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/oses&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44ZcugOKkj@KBlRikOhxKijwAAAQA&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[4] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/oses&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=3&c=33ZcugOKkj@KBlRikOhxKijwAAAQA&t=ct%3Dns%26unitnum%3D3%26raptor%3Deagle%26pos%3Dmid%26test%3D0

[5] https://learn.microsoft.com/en-us/windows-hardware/design/minimum/supported/windows-11-supported-intel-processors

[6] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/oses&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44ZcugOKkj@KBlRikOhxKijwAAAQA&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[7] https://www.theregister.com/2024/02/01/windows_10_users_errors_hardware/

[8] https://www.theregister.com/2024/02/09/copilot_notepad/

[9] https://www.theregister.com/2024/02/09/windows_10_upgrade_closure/

[10] https://www.theregister.com/2024/02/08/windows_11_sudo/

[11] https://www.theregister.com/2024/02/02/windows_12_windows_11_24h2/

[12] https://whitepapers.theregister.com/